This invention relates to torque transmitting couplings and in particular to an improved key and a key joint.
Keys and key joints for transmitting a torque between a shaft and a hub are well known. A standard key is usually solid and is of a rectangular parallelepiped shape. A key joint incorporates the key between a shaft and a hub with a portion of the key in a keyway in the shaft and the other portion in a keyway in the hub. When a torque is applied to either the shaft or the hub, the shaft and the hub undergo different amounts of elastic deformation. At the key joint however, the key tends to equalize the strains in the shaft and in the hub. This causes a high stress, or stress concentration, in the shaft and hub at the key joint and especially at the ends of the key where the different strains merge into one another. Errors in assembling the key joint can further aggravate the stress concentration problem.
Some prior art keys are designed to ease the transition between the different strains of the shaft and the hub at the ends of the key. One such key is that disclosed in Foppl U.S. Pat. No. 2,041,123 issued May 19, 1936 in which a key is disclosed having tear drop shaped bores adjacent to arcuately shaped ends. Blue et al U.S. Pat. No. 3,920,343 discloses another key having arcuate ends which are centrally bored from top to bottom.
A problem with the prior art keys having modified ends is that they are difficult and expensive to manufacture. Standard keys are usually manufactured simply by being cut and deburred from standard bar stock. The prior art keys having modified ends would require at least two operations in addition to cutting and deburring. The Foppl design would require the ends of the key to be formed, presumably by grinding, into an arcuate shape and then it would require a sophisticated machining process to form the tear drop shaped bores. The Blue et al design would also require the ends of the keys to be made arcuate and then would require a boring operation for each end. Therefore, a need exists for a key which minimizes stress concentrations at the key joint and is easy and inexpensive to manufacture.